The present invention relates to data printers which output and print digital image data in a computer, a facsimile terminal, etc., and more particularly to the control of the quality of images printed by the printers.
Recently, printers using an electrophotographic process, electrostatic printers using an electrostatic recording process or thermal printers using a thermal recording process are generally used to output and record digital print data from computers, facsimile terminals, etc. The printers using the electrophotographic process include a laser printer using laser beam scanning and exposing techniques, an LED printer using an LED head including an array of LEDs one for each recorded dot as an exposure source, and a liquid crystal printer using a liquid crystal head having an array of liquid crystal shutters one for each recorded dot. The general principles, structures and operations of these printers are disclosed in a reference entitled "Essentials of Development and Application of Non-Impact Printers", Electronics Essentials Series, No. 14, first edition, Kabushiki Kaisha Nippon Kogyo Gijutu Center, Aug. 15, 1985, pages 127, 141 and 155. The problems with the conventional techniques will now be described by taking as an example one of the laser printers disclosed therein.
FIG. 1 is a schematic of a general laser printer using a semiconductor laser as an exposure source. The laser 1 modulates its rays in an on-off control manner via the driver 2 in accordance with an image signal. The modulated laser rays are collimated by a collimator lens 3, and the collimated rays are scanned deflectively over a photosensitive body 5 (hereinafter referred also to as a photoconductive recording medium) used as a recording medium by a polygon deflector 4. The photosensitive body 5 rotates in the direction perpendicular to the direction of scan of the laser beam 6. The laser beam 6 is focused by a focusing lens 7 onto the photosensitive drum 5 so as to have an appropriate beam spot diameter. An electrostatic latent image is formed on the photosensitive drum 5 by such scanning exposure. Thereafter, a visual image constituted usually by black and white bi-level dots through developing, transferring and fixing processes is printed and output on recording paper.
In this conventional printer, it is necessary to control the print density of the visual image on the recording paper invariably at an appropriate level. In addition, it is necessary to provide an optimal print image quality, for example, to beforehand equalize the widths of a horizontal and a vertical lines used for rules. To this end, various control systems are studied and put to practical use. As disclosed in JP-B-62-50818, there is a density control method of optically sensing the print density of a visual image on a photosensitive body or recording paper with a density sensor and controlling the print density in accordance with the output of the sensor.
Only by such control of the print density at the given appropriate level, however, the respective components of the laser printer using the electrophotographic process are likely to be changed due to aging and in characteristic depending on ambient conditions, so that the quality of the print image would be undesirably different from the initial set one. For example, even if the light emission modulation time for a laser in which the intensity of the exposing beam and the print width of one dot are determined is adjusted such that the horizontal and vertical line widths are equal initially, the widths of both the lines may be changed due to aging and changes in the ambient temperature because of the deterioration of the output characteristic of the semiconductor laser and a change in the exposure surface potential attenuation characteristic of the photosensitive body due to temperature. For example, the gradient of a curve indicative of the output characteristic of the semiconductor laser is reduced with a time interval in which the laser is used, as shown in FIG. 2a. Therefore, even if the intensity of the exposure beam and the light emission modulation time of the laser are fixed, the emission waveform of the laser becomes blunt as shown in FIG. 2b. In such a case, the width dv of a horizontal line does not change because printing is performed by the continuous emission of the laser, but the width of a vertical line associated intimately with the light emission waveform of the laser is thinned (dH-dH') with the deterioration of the laser light emission characteristic. With a change in the photosensitive body characteristic due to temperature, as mentioned above, a change in the width of the vertical line is small, but the horizontal line is thickened or thinned depending on the temperature level.
As just described above, the print density is controlled to a predetermined density level in the conventional printer, but there are no measures for protecting the initial appropriate image quality from the influence of changes in the environments where the printer is used, for example, changes in the ambient temperature, and aging due to long time use, and thus the image quality would change with time. These problems are likely to appear especially in the gradation characteristic used for recording a halftone image. For example, even if the gradation characteristic curve is initially set so as to be linear as shown by a characteristic curve a in FIG. 3, it may change to a characteristic curve b or c if there are changes in the quality of a print image, as mentioned above, and thus the output image would change undesirably each it is printed.
A similar problem would arise in a liquid crystal printer as well as an LED printer of a recording system in which an array of heads is used. In addition, a similar problem would also arise in a data printer such as an electrostatic printer and a thermal printer different in system from the liquid crystal and LED printers.